1. Cross-Reference to Related Application
Copending patent application, by the same applicant and assigned to the same assignee, discloses related subject matter, entitled "Apparatus for Applying an Electric Field", U.S. Ser. No. 07/722,180, filed Oct. 7, 1991.
2. Field of the Invention
The present invention relates to a method for generating a plasma in a mold tool. More specifically, the invention relates to a method of treating a polymeric molded article with the plasma to make it more receptive, to coatings or to apply a coating on the article, for example painting.
3. Description of Related Art
It is known to expose articles to a plasma to change the surface composition of the article. Such exposure is generally termed "plasma treatment". Low pressure plasma treatment is useful for stabilizing a molded article containing a plasticizer and for making the article more receptive to coatings such as paints, lacquers, adhesives, films, etc. It is also useful to plasma-treat an article for antistatic purposes. Generally, low pressure plasma treatment involves placing the article to be treated in a chamber and reducing the chamber pressure to between 1-0.01 Torr. An ionizable gas is introduced into the chamber and the gas is transformed into a plasma. One method of creating the plasma is by passing the gas through an electric field created between two electrodes. The electric field excites the gas and ionizes the individual molecules of the gas to form a plasma. The plasma treats all exposed surfaces of the articles within the chamber. This method of treating one or more articles in a chamber is known as batch processing. The chamber must be stocked with articles for plasma treatment, and then the entire chamber evacuated to a relatively low pressure. The larger the chamber, the longer the evacuation time and more expensive and complex are the individual components necessary to draw vacuum and create a plasma. One such example of a batch processing plasma treatment is taught and described in U.S. Pat. No. 4,107,049 (Sano et al).
Surface modification, specifically for polymeric substrates, is useful for enhancing the adhesion properties of the substrate. Polymeric material such as polypropylene and polyethylene contain fully hydrogenated surfaces which are not polar and therefore not receptive to coatings. By exposing this surface to a plasma, various constituent molecules having more polar species can be grafted onto the polymer. These polar species enhance the adhesion of the surface to various coatings. Various surface modifications are described in Liebel and Bischoff, "Pretreatment of Plastics Surfaces in the Low-Pressure Plasma", Kunststaffe German Plastics, 1987; and in Takahashi, Fukuta, and Kaneko, "Plasma Treatment for Painting of Polypropylene Bumper", SAE 850320, both incorporated herein by reference.
Previous attempts to graft adhesion-promoting molecules onto a substrate required batch or semi-batch processing. Batch processing has previously been described as taught in Sano et al. Semi-batch processing consists of a series of chambers, each containing articles to be treated. The chambers are evacuated in series. The plasma is produced by a plasma generating means outside of the chambers. Once an individual chamber has been evacuated to the proper pressure, the plasma is introduced to treat the articles therein. Semi-batch processing permits the plasma to be created in one location while the chamber containing the article to be treated is evacuated. Semi-batch processing permits a slight increase in speed over batch processing. Because both batch and semi-batch processes use chambers having a volume far in excess of that required to create a plasma, these processes are still relatively inefficient.
It is an object of the present invention to provide a method for manufacturing a plasma treated article without using a large volume vacuum chamber.
It is a further object of the present invention to provide an efficient, low-cost method of plasma treating a molded article using the molding tools.
It is another object of the present invention to provide a method of injection molding and plasma treating a molded article using a single mold tool.
It is still another object of the present invention to provide a low-cost method for manufacturing an article receptive to a coating.